Rotating shafts such as those used in gas turbine engines require bearings to react radial and thrust loads in relatively moving or rotating components.
Where light weight and minimum power loss from friction are required, rolling element bearings are common, and may be used to react both radial and thrust loads. Examples of rolling element bearings include ball bearings and roller bearings.
In some applications, where space is limited, multiple thrust bearing rows can be stacked to share the thrust load, as shown in FIG. 1.
Referring to FIG. 1, a prior thrust bearing assembly 1 comprises a pair of inner bearing races 2, 4 which each engage against a first side of respective first and second sets of rolling elements 3, 5. The assembly 1 also comprises an outer bearing race 6 which engages against a second side of the first and rolling elements 3, 5. The inner bearing races 2, 4 are coupled to a first shaft 7, and the outer bearing race 6 is coupled to a second shaft 8, or a stationary component. The bearing assembly 1 thereby rotatably couples the first and second shafts 7, 8 together. In use, a load is applied in an axial direction X to the outer bearing race 6 by the shaft 8, and is transferred to the inner bearing races 2, 4 via the rolling elements 3, 5. The first, and second inner bearing races 2, 4 define a pair of load paths 9a, 9b extending between the first and second shafts 7, 8 through respective bearing races 2, 4. The load is thereby shared between the first and second bearing races 2, 4. Such an assembly is known as a “stacked bearing”. Further bearing sets and bearing races can be added to either end, such that the load can be shared by three or more bearing sets. Such an arrangement may be capable of reacting relatively large loads, while having a relatively small radial width.
One problem with prior bearing assemblies such as the stacked bearing assembly described above, is that the geometry of the rolling elements and bearing races must be carefully controlled such that the load is shared relatively equally between the pairs of races and rolling elements. Small variations (such as of the order of a few microns in some cases) in the geometry of the rolling elements or bearing races can lead to one component taking more of the load relative to the other. Furthermore, the materials of the rolling elements or bearing races may expand in use due to heating, which further exacerbates the geometrical variations, leading to a “runaway” effect, in which one component takes progressively more of the load, which may eventually lead to bearing failure. On the other hand, underloading of one set of rolling elements may result in “skidding” of that set, which may result in damage, debris release and bearing failure. Such bearings are also relatively difficult to supply with lubricating or cooling fluids.
Stacked bearing arrangements including hydraulic or pneumatic pistons have been suggested to address the above issues, such as that described for example in U.S. Pat. No. 8,083,472. However, such arrangements are not sufficiently robust for use in high temperature, high vibration environments, or for use in safety critical applications, such as compressor and fan shafts for gas turbine engines.
Gas turbine engines employ at least one shaft for transferring mechanical power between a turbine and a compressor. FIG. 2 shows a turbofan engine 10 comprising an air intake 12 and a propulsive fan 14 that generates two airflows A and B. The gas turbine engine 10 comprises, in axial flow A, an intermediate pressure compressor 16, a high pressure compressor 18, a combustor 20, a high pressure turbine 22, an intermediate pressure turbine 24, a low pressure turbine 26 and an exhaust nozzle 28. A nacelle 30 surrounds the gas turbine engine 10 and defines, in axial flow B, a bypass duct 32. The engine 10 is supported by outlet guide vanes (OGVs) 39, which extend between the engine 10 and nacelle 30, and also serve to remove exit swirl from the fan 14.
The engine 10 further comprises a high pressure shaft 34 connecting the high pressure compressor 18 and high pressure turbine 20, an intermediate pressure shaft 36 connecting the intermediate pressure compressor 16 and intermediate pressure turbine 24, and a fan shaft 38 connecting the fan 14 and low pressure turbine 26. Each shaft 34, 36, 38 is mounted by one or more respective bearing assemblies 37, either to another shaft, or to a static structure of the engine 10. Each bearing assembly 37 must usually constrain both radial (i.e. vibration) and axial (i.e. thrust) loads.
In particular, where the fan 14 loses a fan blade (known in the art as a “blade off”), the fan shaft bearing 37 must constrain significant radial loads for a period of time as the engine 10 is shut down. In one prior arrangement, additional means are provided to allow the fan to rotate in an eccentric state without overloading the engine structure. Such means may include fusible elements that allow the fan to run on additional catcher bearings, or further articulation in the bearing supports provided by universal joints.
The present invention describes a bearing arrangement and a gas turbine engine comprising a bearing arrangement which seeks to overcome some or all of the above problems.